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1. Field of the Invention
The apparatus of the present invention relates to a system and method for heating drilling, completion, and/or stimulation fluids including acidizing liquids xe2x80x9cfluidsxe2x80x9d but not limited thereto. More particularly, the present invention relates to a system and method for heating drilling and/or completion fluids entering or exiting a wellbore in order to enhance the stability of the well bore improve the capability of the fluids to retrieve solids from the borehole and enhance production.
2. General Background of the Invention
In the rotary drilling operation within the Exploration and Production industry there is a fluid that is universally called xe2x80x9cdrilling fluids/mudxe2x80x9d. The drilling fluid is circulated from a storage system xe2x80x9cmud pitsxe2x80x9d on the surface of a drilling/production vessel downward through the drill pipe and out of the apertures xe2x80x9cjetsxe2x80x9d of the drill bit and upward within the interior of the casing side of the wellbore and to the borehole to the surface. Returning to the surface, the drilling fluid carries xe2x80x9cdrilled cuttingsxe2x80x9d generated as the drill bit cuts the earthen formation and carries the drilled cuttings to the surface in the mud. At the surface the mud is treated through a variety of xe2x80x9csolids controlxe2x80x9d equipment, i.e.: gumbo chain, shale shakers, de-sanders, de-silters, centrifuges, and cuttings dryers, etc. removing the drilled cuttings and ultimately returning the mud to the mud pits for reuse.
Today it is recognized that a drilling fluid has a number of important functions but not limited to the following:
1. Removes cuttings from the bottom of the hole and carries them to the surface
2. Holds cuttings and weight material in suspension when circulation is interrupted
3. Releases sand and cuttings at the surface
4. Walls the hole with an impermeable cake
5. Minimizes adverse effects upon the formation
6. Cools and lubricates the bit and drill string
7. Supports part of the weight of the drill stem and casing
8. Controls subsurface pressure
9. Transmits hydraulic horsepower
10. Maximizes downhole information obtained.
Removing cuttings from below the drill bit is still a crucial function of a drilling fluid. The circulatory fluid rising from the bottom of the well bore carries the cuttings toward the surface. Under the influence of gravity, these cuttings tend to fall through the ascending fluid. This is known as slip velocity. The slip velocity will depend upon the viscosity (thickness) and density of the fluid. The thicker the fluid, the lower the slip velocities. The more dense the fluid, the lower the slip velocity. For effective cuttings removal, the fluid velocity must be high enough to overcome the slip velocity of the cuttings. This means that fluid velocity can be lowered in a highly viscous (thick) or very dense fluid and cuttings still effectively removed from the well bore. The density of a fluid is determined by other factors and is not usually considered a factor in hole cleaning; therefore we limit adjustment of hole cleaning properties to viscosity and velocity adjustments to the drilling fluid. The viscosity desired will depend upon the desired hydraulics and the size of the cuttings contained in the fluid. The velocity will depend on several factorsxe2x80x94the pump (capacity, speed, efficiency), the drill pipe size and the size of the bore hole.
The velocity of a fluid will determine its flow characteristics, or flow profile. There are five stages, or different profiles, for a drilling fluid: 1) no flow, 2) plug flow, 3) transition, 4) laminar, 5) turbulent. The ideal velocity is one that will achieve laminar (or streamline) flow because it provides the maximum cuttings removal without eroding the well bore. On the other hand, turbulent flow (resulting from too high a velocity or too low fluid viscosity) not only requires more horsepower but can cause excessive hole erosion and undesirable hole enlargement. The proper combination of velocity and viscosity is a must for the right hydraulics and efficient hole cleaning. Cuttings will have a tendency to collect at points of low fluid velocity in the well bore annulus. These areas are found in washouts and where the drill pipe rests against the wall of the well bore. To that end, it is a good practice to rotate the drill string while just circulating to clean the hole, as this will help keep the cuttings in the main flow of the fluid and not allow them to gather next to the wall or pipe.
When circulation is interrupted, the slip velocities of the cuttings will cause them to fall back to the bottom of the hole unless the drilling fluid can suspend the cuttings with its gel strength. Upon resumption of circulation, the fluid reverts back to its fluid state and carries the cuttings to the surface. This ability of developing gel properties while static and then becoming fluid again when pumped is called the thixotropic property of a drilling fluid. The ability of thickening at low velocities and thinning at high velocities is called the shear-thinning property of a drilling fluid. The magnitude of gel strengths and shear-thinning ability of a drilling fluid will depend upon the concentration and quality of clay solids in the fluid system.
Once the cuttings are out of the hole, they must be removed from the system to keep from being re-circulated. This can be done by using a low-gel-strength fluid and allowing the cuttings to settle out. The cuttings can also be removed by mechanical means such as gumbo chains, shale shakers, de-sanders or de-silters, centrifuges and cuttings dryers. If the cuttings are re-circulated, they are subjected to further grinding action and abrasion. As the cuttings become smaller, they become harder to remove and tend to remain in the system.
Over a period of time, this will cause undesirable Theological properties resulting in high chemical treatment costs and also slower penetration rates that result in higher well costs.
Considerable heat is generated by friction at the bit and where the drill string is in contact with the formation. This heat must be absorbed by the circulating fluid so that it can be transmitted to the surface and dissipated. The fluid also works to lubricate the bit and drill string. If additional lubricity is needed, there are several lubricating products which can be added to the system. The lubricity of the fluid helps to decrease torque, increase bit life, reduce pump pressure and reduce bit balling.
A drilling fluid will deposit a filter cake on the wall of the well bore. This wall cake helps protect the formation by retarding the passage of mud filtrate into it. The higher the permeability of a formation, the greater its ability to accept and receive large volumes of mud filtrate. Therefore, the nature of this filter cake will have a direct effect on such problems as formation damage, sloughing and caving, tight hole and stuck pipe. The type of wall cake is determined by the quantity and quality of particles in the mud system.
Performance advantage is the most significant, because borehole stability is the number one reason given for drilling fluids selection. Water based fluids can be engineered to perform (or out-perform if environmental factors/expense warrant) as well as oil based drilling fluid in all areas but shale stability. Hydrating shale, which makes up 75% of most marine depositional basins, is the main cause of lost hole and down hole drilling problems. Marine shale is composed of clays containing smectites and illites. Although illite is not as active (expanding, swelling) as the smectite group, illite will expand or destabilize over time. Oil based drilling fluid, with oil as the continuous phase and water tightly emulsified as droplets, does not provide a hydrating medium for the active clay content of marine shale""s.
The low oil filtrate and excellent lubricity characteristics aid in reducing differential sticking in highly permeable formations and high angle holes.
An oil mud will not dissolve water soluble formations such as salt or gypsum; also provides stability from acid gas bearing formations.
The nonconductive, external phase of an oil mud prevents maximum protection for drill pipe and casing.
The permeability of producing sands is not reduced since the filtrate will not cause swelling and dispersion of hydrateable clays that are in the pores of the sand.
The proper restraint of formation pressures depends upon the density (weight) of the fluid. Abnormal high pressures can be controlled by the weighting up of a fluid with the addition of certain materials Sybarite is the most common). In some cases, however, a fluid can become too heavy and hydraulically fracture a formation, causing lost circulation.
The drilling fluid is the medium which transmits available hydraulic horsepower to the system. This horsepower is needed to move the fluid through the surface system, down the drill string, through the bit, up the annulus (the space between the hole wall and the drill pipe), through the pits and back to the suction pump. Fluid flowing from the bit nozzles exerts a jetting action that keeps the face of the hole and the teeth edge of the bit clear of the cuttings. The horsepower required to move the mud through the remaining system should be minimized in order to maximize horsepower at the bit. The heavier a fluid becomes, the greater the horsepower that is required to move it through the system. This results in less horsepower at the bit and slower penetration rates.
When a drilling fluid is controlled and properly maintained, it not only insures proper formation protection, optimum penetration rates, greater well production and lower equipment wear, but can also, within given parameters maximize downhole information. A hole is drilled into the ground to 1) gather information on those rocks penetrated by well bore, and 2) find and recover usable fluids. A properly controlled drilling fluid is necessary not only to recover adequate rock cuttings for their analysis and study, but also to safely control subsurface pressures, optimize penetration rates for controlling drilling costs, minimize formation damage and therefore maximize well productivity.
The present invention relates to the heating of drilling, completion, and/or stimulation fluids including acidizing liquids but not limited to entering or exiting the well bore. This process will be utilized to pre-treat drilling/completion fluids entering the well bore as well as post treatment of the fluids exiting the well bore. The primary objective of the invention is to overcome colder conditions in the drilling fluid that will complicate well bore stability concerns and decreased drilled solids removal efficiencies. The functions of the drilling fluids, as listed earlier, will become enhanced by heating the drilling fluids as well as adding a significant economic savings for the user.
To carry out the method of the invention there will be provided a principal heat exchange vessel; i.e., plate and frame, shell and tube, fintube, spiral coil, platecoil and embossed immersion heat transfer panels, a first inlet for introducing heat transfer media i.e., heat transfer oil, xe2x80x9chot oilxe2x80x9d heated air, or steam into fluid flow lines within the vessel; an outlet for flowing the heat transfer media from the vessel; a second inlet for introducing drilling and/or completion fluids returning from or returning to the well bore for receiving heat from the heat transfer media within the fluid flow lines in the vessel; an outlet for flowing the heated downhole fluids from the vessel to be returned down the borehole; a heater i.e., diesel xe2x80x9cpropane or natural gasxe2x80x9d fired, steam, hot air, electrical or the like for heating the heat transfer media to a desired temperature before returning the heat transfer media to the heat exchange vessel. Optimally, the drilling or completion fluid would be heated to 50 to 60F above ambient temperature.